Solar collector panels capture heat from the sun and transfer the heat directly to a space to be heated, such as the interior of a building, or to a heat storage device. In general, solar collector panels comprise an enclosed space, glazing oriented to the sun, a plate which absorbs solar radiation and converts it into heat, and intake and outflow passages for a circulating heat-transfer fluid. A panel is said to be air-based if the heat transfer fluid is air. The panel is said to be liquid-based if the heat-transfer fluid is liquid. The system as a whole is said to be active if it utilizes an extrinsic device for circulating the fluid, rather than relying on natural convection.
Air-based panels can be used by themselves to bring heated air directly into a living space, or in conjunction with a storage mechanism which retains heat energy for release at a later time. In a system intended to provide a significant portion of a building's heating needs on a continuous basis, some form of storage is required. Without storage, living-space air becomes uncomfortably hot during the day, requiring venting and thus energy wastage. Furthermore, little energy is retained for release at night or on cloudy days when the panels are not in operation.
Panels have varying levels of "efficiency". This term is used in a very general sense in the art to describe the percentage of solar energy, out of that theoretically possible, which is actually delivered to the interior of a building or a storage mechanism. When not in operation, panels lose no energy, being remote from the living space. However, they do lose a significant amount of energy while in operation. Glazing reflects some sunlight, particularly at acute angles of incidence ("reflection loss"); it also absorbs and re-radiates some radiation ("re-radiation loss"). Far more important, even as panels gain heat from the sun, they also lose heat through the glazing to the outdoor air ("heat loss"), reducing the amount of heat delivered to the interior of a building or a storage mechanism ("net heat yield"). The rate of heat loss is directly proportional to the difference between the panel's air temperature and the temperature of the outdoor air.
Through minimization of panel air temperature, a panel can be made more "efficient". A simple way to do this is to have a large fan and a differential thermostat setting which result in evacuation of panel air as soon as it becomes, for example, 1.degree. F. hotter than living-space air. "Efficient" panels are correspondingly associated with low delivered-air temperature, since it is the very device of lowering the temperature which makes them efficient. However, this "efficiency" is to a great extent illusory, since low-temperature delivery is incompatible with storage mechanisms, whose storage capacity is dependent on the temperature to which they can be heated. When storage is desired, the system must be designed to deliver very hot air, which promotes panel heat loss and lowers panel "efficiency" but does get the delivered heat into storage. The various very-hot-air systems have their own drawbacks aside from lower net heat yield, such as, a shorter operating day, more expensive materials and assembly, and greater weight which complicates installation.
Thus, with existing panel designs, a user must choose between low-cost, high net heat yield panels without the possibility of storage, or higher-cost, lower heat yield panels which permit storage.
As previously discussed, solar systems intended to provide a large part of a building's heat on a continuous basis require some form of heat energy storage. Storage mechanisms can be classified according to the type of energy-retentive material used--liquids, rocks, and phase-change materials. Among the more important criteria for selection or design of a storage system are: installed cost, volume of occupied space in relation to heat-storage capacity, and compatibility with different types of collector panels.
Phase-change materials occupy the least space and can be used with both liquid- and air-based panels given the proper heat-transferring devices. However, they must be hermetically-sealed in pressure-resistant containers and are extremely expensive.
Rock or masonary storage in a bin is relatively inexpensive but occupies a very large amount of space and is generally impractical for installation in pre-existing structures while imposing serious architectural constraints on new ones. It's main advantage is compatibility with air-based panels, which (even if of the very-hot-air variety) are much more efficient and much cheaper to fabricate and install than liquid-based panels.
Liquid storage--typically a water tank--is also inexpensive and typically occupies about one-quarter the space of a rock-bin for a given storage capacity. Liquid storage devices are somewhat easier to install and fill than rock-bin devices. Because of these characteristics, liquid storage devices are comparatively easier to retrofit into existing structures and imposes few constraints on new-structue design. Water tanks would be nearly ideal storage if not for the fact that heat is difficult to transfer from air to water and thus they have up to now necessitated the use of liquid-based panels. However, liquid-based panels are themselves expensive, complicated, and heavy; need expensive plumbing and control devices; are prone to mechanical breakdown and freezing damage; and yield less heat than air-based panels.
Accordingly, there is a need in the art for a solar heating system wherein the user does not have to choose between low-cost, high net yield collector panels without the possibility of thermal energy storage or higher-cost, lower yield panels which permit storage. There is also a need in the art for a low-cost thermal energy storage mechanism which occupies a minimum of space while storing a maximum of heat, and which is capatable with low-cost, high net yield air-based solar collector panel.